The present invention relates to an apparatus and method for bonding an optical element to a support and, more particularly, to such bonding which is unaffected by any differential coefficients of thermal expansion of the optical element, the support and the material bonding the element to the support.
Optical glass and like optical elements are conventionally bonded to a support by bonding materials. Typically, bonding materials have a coefficient of thermal expansion which is approximately ten times higher than that of the optical element of the substrate to which the element is bonded. As a consequence, temperature changes cause differential expansion of the bond and thereby result in optical shifts in the optical element. In addition, if the bond interface is not exactly uniform in thickness, the thicker part expands more than the thinner part, to create a wedge which results further in an optical shift.
Sometimes, even if the bond thicknesses are made precisely uniform throughout the bonding area, the bonding material buckles, which causes the optical element to tilt with respect to its support, and produce an optical shift.
Such thermally induced optical shifts are highly undesirable as causing optical misalignment in precise optical equipment.
One common method of minimizing such optical shifts and optical misalignment requires the selection of bonding materials which tend to minimize such shifts. In addition, the bond thickness is made as small as possible so that its effect to produce an optical shift is minimized. Finally, some techniques employ a boresight maneuver immediately prior to use of the equipment, first to measure the optical shift and then to correct it either electronically or physically by tilting mirrors, in order that the optical shift be nulled out. Such approaches significantly affect the overall cost and complexity of the system.
An additional problem occurs when a system includes two or more optical elements, for example, prisms, which are to be aligned one with respect to the other. Positioning of such multiple image prisms requires that each prism be positioned precisely with respect to other prisms. Conventionally, this requires some form of support and the attendant bonding of the prisms thereto. Such bonding techniques, such as referred to above, create difficulties in precise alignment. Typically, the prisms are aligned one with respect to the other in a holding fixture that includes supporting the prism frame. Once the prisms are correctly positioned with respect to the frame, the bondinq material is used to bond the prisms to the support structure. Because the prism surfaces are not necessarily parallel to the walls of the frame, the bond thickness will vary over the bond area, with the result that the prism aliqnment shifts with temperature as a result of the wedge and temperature buckling effects.
The invention described in U.S. Pat. No. 4,857,130 issued 15 Aug. 1989, and assigned to the same assignee of this document, is eminently successful in avoiding- and overcoming the above and other problems. However, when aluminum, for example, was used as the rail material and the assembly was subjected to a low temperature such as -70.degree. F., it was discovered that glass was pulled and torn from the prism where the rail was bonded the prism, because of the differential coefficients of expansion of aluminum and glass.
A similar problem was observed with respect to the normal bond spot procedure, prior to the invention disclosed in above referenced U.S. Pat. No. 4,857,130, in which the differential expansion between the three materials, used with a bond spot, resulted in an increase in glass surface stress. The rail bond plug approach, disclosed in U.S. Pat. No. 4,857,130, added to the glass stress over the normal bond spot stress, due to the differential expansion between the bond epoxy and the linear expansion at the feet of the plug metal. Of even more importance, as found by experimentation, any lateral differential expansion between the optical glass element and the plug material of aluminum was prevented by the epoxy bridge formed at the bottom of each foot where it contacted the glass surface. This contact point normally would slip with temperature variation but the epoxy bridge, which has a modulus of over 10.sup.6 psi at -70.degree. F., prevented the slippage. This resulted in a build up of stress in the glass to a point where the glass fractured at the contact line of each metal foot and the optical surface.
Before the underlying cause of the glass breakage was determined, it was the prevailing theory of applicant that differential contraction between the rail plug foot material and the bond material was the cause for such fracture.
The initial approach to reducing the differential coefficient of thermal expansion of the material adjacent to the three feet of the bond plug was the addition of a low coefficient of thermal expansion filler to the epoxy. Reducing the overall coefficient of thermal expansion of the epoxy filler mixture by a factor of three, however, did not have any appreciable effect; about the same amount of glass pulling resulted when the assembly was subjected to the -70.degree. F. environment.
The aluminum plug was then replaced by one of steel, while using the same epoxy without filter. Although this modification was expected to increase the differential stress on the glass with the result of increased glass pulling, to the contrary, the result was exactly the opposite in that no glass was pulled even after the assembly was recycled 5 times to an environment of -70.degree. F.
The unexpected result, that the use of the steel plug did not cause glass pulling, provided the clue as to the true cause for the excessive stress build up on the glass, and suggested a solution which resulted in the present invention. The use of the steel allowed the coefficient of thermal expansion of the plug material and the glass to be essentially matched, resulting in a very small lateral strain motion of the base of each plug foot with respect to the glass at -70.degree. F. Because aluminum has a coefficient of thermal expansion of about twice that of glass, the differential expansion between these materials resulted in a much larger relative lateral strain motion. As the modulus of the epoxy approaches 10.sup.6 psi at -70.degree. F., the epoxy bridge between the base of each foot and the glass transforms the above strain into a glass stress with the result that aluminum plugs cause glass pulling and steel plugs do not.
Thus, the discovery of the cause of the glass pulling and the realization, that plugs of aluminum must be used to match the coefficient of thermal expansion of the prism support structure, provided the solution, as embodied in the present invention.